Nembus

Nembus is a minor entry lying toward the boundary between Andromeda and Perseus, a crowded and much-observed part of the northern sky that holds the Andromeda Galaxy on one side and, on the other, the Perseus Double Cluster and the famous eclipsing variable Algol. The name is not securely traceable to any of the main naming traditions, and forms of it appear in the older star lists with variant spellings, a common condition among the minor entries. That instability is worth stating plainly rather than covering over: the fixed-star catalogues used in astrology were assembled by copying from earlier catalogues, and names that were already uncertain in one compilation were reproduced in the next. No traditional planetary nature is recorded for this star, and it carries no interpretive tradition of its own.

The crowded field between Andromeda and Perseus is exactly the kind of region where nineteenth-century compilers, working from several manuscript star-lists that did not always agree, could misattach a name to the wrong star entirely; Edward Ball Knobel's careful 1895 edition of the Al-Achsasi al-Mouakket catalogue was in part an attempt to sort out precisely this kind of confusion by returning to an original Arabic source rather than a chain of Latin copies of copies.

The atlas records this star at Taurus 12 degrees 26 minutes, projected from Andromeda, and notes that it stands near the border with Perseus.

That border is more definite than it sounds and more recent. Constellation boundaries were not fixed until the International Astronomical Union adopted Eugene Delporte's scheme in 1930, drawing them as arcs of right ascension and declination for the epoch 1875. Before that a star near a boundary genuinely belonged to whichever figure an atlas maker chose to draw it into, and old lists disagree. So a star described as being on the edge of Andromeda and Perseus is being described by a convention less than a century old, laid over names many centuries older.

The zodiacal longitude is a separate projection again. Neither Andromeda nor Perseus touches the ecliptic; both lie well north of it, and the sign given here is where a line dropped from the star to the ecliptic falls, not a region of sky the star occupies.

Precession dates the figure. A fixed star advances about fifty seconds of arc a year, one degree in roughly seventy two years, so taking the recorded value for epoch 2000 this star entered Taurus around the beginning of the twelfth century and will reach Gemini in something over twelve hundred years.